System and method for long baseline accelerometer/GNSS navigation

ABSTRACT

A system and method for providing location information using a long baseline accelerometer/GNSS system. A first set of accelerometers is operatively associated with the first GNSS antenna while a second set of accelerometers is operatively associated with a second (or more) GNSS antenna. The multiple assemblies are separated by predefined distances and held rigid to each other. Accelerometer data is combined with the GNSS data to provide improved navigation and location information.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to global navigation satellite system(GNSS) antenna navigation systems and, more particularly, to pairedinertial motion unit (IMU) and GNSS navigation systems.

Background Information

Micro-electromechanical systems (MEMS) based gyroscopes are generallyrelatively expensive and can be highly inaccurate due to, e.g., biasesand/or poor stability. Accelerometers for use in inertial motion units(IMUs) have been examined as a possibility for use in GNSS/INS systems;however, they lack the accuracy due to the short baseline between thepairs of accelerometers when combined in a compact unit for use in aconventional IMU. Conventional MEMS gyroscopic systems often lack thenecessary accuracy for modern navigation systems requirements. Further,their relatively high cost often places the use of such MEMS gyroscopesat price points that are unreasonable and/or unfeasible for manynavigation applications.

SUMMARY OF THE INVENTION

The disadvantages of the prior art are overcome by combining anaccelerometer triad with a GNSS antenna using a long baseline GNSSvector (2-D or 3-D) system. An exemplary accelerometer triad comprisesof three orthogonally oriented accelerometers that are arranged toenable measurement of yaw, pitch and roll of the accelerometer triad.Illustratively, a plurality of antenna/accelerometer triad units areattached to a rigid frame with a reasonable antenna separation, e.g., onthe order of decimeters. This relatively long baseline length increasesthe angular rate sensitivity from the accelerometers as well as from theGNSS. The increased angular rate sensitivity provides a better ratestability and performance over equivalently priced gyroscopes. Aninertial navigation system performs a double integration of the measuredaccelerations between GNSS solutions. By utilizing two or more of thecombination of accelerometer triad/GNSS antennas, any combination ofrotational rate observations may be obtained, including up to sixdegrees of freedom (DOF). By determining the specific forces androtation acting on the rigid body, the system may determine the fullposition, velocity and attitude navigation solution.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further advantages of the present invention may be furtherdescribed in relation to the accompanying drawings in which likereference numerals indicate identical or functionally similar elements:

FIG. 1 is a schematic diagram of an exemplary GNSS antenna andaccelerometer triad system in accordance with an illustrative embodimentof the present invention;

FIG. 2 is a schematic diagram illustrating the exemplary spacing betweenaccelerometer triad/GNSS antenna systems in accordance with anillustrative embodiment of the present invention;

FIG. 3 is a schematic diagram of an exemplary navigation location systemutilizing accelerometer triad/GNSS antenna pairs in accordance with anillustrative embodiment of the present invention; and

FIG. 4 is a flowchart detailing the steps of an exemplary procedure foridentifying location information utilizing accelerometer triad/GNSSantenna pairs in accordance with an illustrative embodiment of thepresent invention.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

FIG. 1 is an exemplary perspective diagram of an illustrativeantenna/accelerometer triad system 100 in accordance with anillustrative embodiment of the present invention. Theantenna/accelerometer system 100 illustratively comprises two GNSSantennas 110 A, B each operatively associated with an accelerometertriad unit 115 A, B that are mounted to a rigid body 105. However, inaccordance with alternative embodiment of the present invention, morethan two GNSS antenna/accelerometer triad units may be utilized. Assuch, the description of two GNSS antenna/accelerometer triad unitsshould be taken as exemplary only. The GNSS antennas 110A, B maycomprise conventional GNSS antennas that are commonly utilized by thoseskilled in the art. The accelerometer triad units 115A, B illustrativelycomprise three accelerometers arranged so that they measure yaw, pitch,and roll rates as well as absolute pitch and roll of the rigid body. Theaccelerometer triad units 115A, B may illustratively comprise a singleunit of three accelerometers. However, in alternative embodiments, aplurality of separate accelerometers may be utilized to form theaccelerometer triad unit 115. As such, the description of three separateaccelerometers comprising the triad unit should be taken as exemplaryonly.

Illustratively, the accelerometers are arranged orthogonally so thatthey may measure acceleration in the X, Y and Z axis as well as provideyaw, pitch and roll rate information. In an exemplary embodiment, theymay be arranged along the edges of the GNSS antenna. However, it isexpressly contemplated that the accelerometers may be arranged indiffering configurations. Information received from the GNSS antennas110 and the accelerometer triad units 115 are fed into a receiver unit300, described further below in reference to FIG. 3.

The rigid body 105 may comprise a structural element on which the GNSSantennas and accelerometer triads are mounted. Illustratively, the rigidbody 105 may comprise an element of a vehicle (not shown) on which theGNSS/accelerometer triad units are mounted. For example, the rigid body105 may comprise of the roof of a vehicle that utilizes theGNSS/accelerometer units for navigational information. More generally,the rigid body 105 may comprise any structure that supports the set ofGNSS 110 and accelerometer 115 separated by a predefined distance inaccordance with illustrative embodiments of the present invention. Theset of GNSS 110 and accelerometer triad units 115 needs to be rigid sothat any rotation between the two or more sets of GNSS/accelerometertriads is maintained. Thus, for example, a set mounted on separatevehicles would not be operative. However, sets mounted on a common roofof a vehicle, etc. that provides rotational consistency may be utilizedin accordance with exemplary embodiments of the present invention. Assuch, the description of the rigid body 105 being a separate componentfrom a vehicle, etc. should be taken as exemplary only. More generally,the rigid body 105 comprises any device or construct that supports theset of GNSS/accelerometer triads at a predefined distance apart. Forexample, in alternative embodiments, the GNSS/accelerometer triad unitsmay be located on separate mounts that are a predefined distance awayfrom each other. As noted above, such separate mounts must berotationally linked to each other. That is, there must be a rigid andpersistent relationship between the two sets of mounts to ensurerotational consistency among the steps through various degrees offreedom.

During operation, the system computes a precise baseline vector betweenthe at least two GNSS antennas 110A, B along the rigid body 105 toprovide a two (or three) dimensional attitude solution. Roll and pitchinformation may be computed directly from the accelerometer data bymodeling the gravity vector. The system may then remove the effects ofgravity and other errors to obtain a measurement of the acceleration androtation acting on the system 100. By performing a double integral onthe accelerometer data, update position solutions may be determinedbetween available GNSS solutions.

FIG. 2 is a schematic diagram illustrating the exemplary spacing betweenaccelerometer/antenna systems in accordance with an illustrativeembodiment of the present invention. As shown in FIG. 2, the rigid body105, which is illustratively displayed as a rectangular structuresupports a pair of accelerometer triad units 115A, B separated by adistance d. However, it should be noted that in alternative embodimentsof the present invention, more than two GNSS/accelerometer triad unitsmay be utilized mounted to a three dimensional rigid structure(s).Illustratively, when more than two GNSS antenna/accelerometer units areutilized in alternative embodiments, they may be arranged orthogonally.In accordance with an illustrative embodiment of the present invention,the distance d is on the order of decimeters. However, it should benoted that in alternative embodiments of the present invention, thedistance's order of magnitude may differ. As such, the description of adecimeter order of magnitude separation between accelerometer triadunits should be taken as exemplary only. As will be appreciated by thoseskilled in the art, the required or desired separation may varydepending upon the sensitivity of the accelerometers and/or thefrequencies involved with the GNSS system. For example, more preciseGNSS systems may require a smaller amount of separation. Similarly, moreaccurate accelerometer triads may require less of a separation. Thus,developing a desired separation may be based on design choices based onrequired size, cost, etc.

The system 100 encompasses the rigid body to enable rotational solutionsto be determined based on the two accelerometer triads. Further, abaseline vector may be computed using, e.g., carrier phase observations,between the two GNSS antenna connected to the rigid body 105.

FIG. 3 is an exemplary schematic diagram of an exemplarynavigation/location system 300 in accordance with an illustrativeembodiment of the present invention. Illustratively, the system 300 isembodied as a GNSS subsystem 310 operatively interconnected with an INSsubsystem 305 in accordance with an illustrative embodiment of thepresent invention. The GNSS subsystem 310 and INS subsystem 305 operateunder the control of a processor 315 to calculate the GNSS and INSpositions, as well as appropriate velocity, pitch, yaw and rollinformation. The GNSS subsystem 310 processes satellite signals receivedover antennas 110 A, B. The INS system receives measurements fromaccelerometer triads 115A, B comprising data from the exemplaryorthogonally positioned accelerometers. The INS system may perform amechanization process, described further below, to obtain location androtational information using the accelerometer data. The data from theaccelerometer triads is time tagged by the GNSS clock 320. The GNSS andINS systems can thus reliably interchange position related informationthat is synchronized in time. The two systems are illustrativelyoperated together, through software integration in the processor 315 toenable position and navigation related information to be shared betweenthe two systems. For ease of understanding, the description of theprocessing operation of the two systems are made without specificreference to the processor 315. The system may, in alternativeembodiments, instead include dedicated GNSS and INS sub processors tocommunicate with one another at appropriate times to exchangeinformation that is required to perform the various GNSS and INScalculations operations discussed below. For example, the INS processormay communicate with the GNSS processor when INS data is provided to thesub processor in order to time tag the data with GNSS time. Further, theGNSS sub processor communicates with the INS of processor to provideGNSS position information at the start of measurement intervals and soforth.

At start up, the GNSS system operates in a known manner to acquire thesignals from at least a minimum number of GNSS satellites to calculatepseudo-ranges to the respective satellites. Based on the pseudo-ranges,the GNSS system determines its position relative to the satellites. TheGNSS system may also determine its position relative to a fixedposition-based receiver (not shown) in the use of differentialcorrection measurements generated at the base station. At the same time,the INS system processes the accelerometer data, that is, themeasurements from the various accelerometers to determine inertiallocation/navigation information. The INS system further processes boththe INS data and the GNSS position and associated covariance informationto set up various matrices for a Kalman filter 325. At the start of eachmeasurement interval, the INS subsystem updates the Kalman filter andprovides updated error states to a mechanization process. Themechanization process uses the updated information and the INS data topropagate, over the measurement interval, the inertial position,attitude and velocity with the inertial position and other systemelement errors being controlled with GNSS positions at the start of themeasurement interval.

At startup, the INS system determines which accelerometers are presentand connected to the processor in order to ensure that the INSmeasurements are scaled correctly.

A generic Kalman filter processes estimates a series of parameters thatdescribe and predict behavior of the system. The Kalman filter operateswith a set of state variables that describe errors in the system andassociated variants covariance matrix that describes the currentknowledge level of the states. The Kalman filter maintains an optimalestimate of system errors and associated covariance over time in thepresence of external measurements to the use of propagation and updatingprocesses. To propagate the state and covariance from some past time tothe current state what time, the Kalman filter propagation and usesknowledge of the state dynamic behavior determined from the physics ofthe system and the stochastic characteristics of the system over time.Kalman filter updates use the linear relationship between the state andobservation vectors in conjunction with the covariance matrices relatedto those factors to determine corrections to both the state sector inthe state covariance vector.

In accordance with an illustrative embodiment of the present invention,accelerometer data is collected and utilized to compute pitch and rollinformation by the modeling of the gravity vector. Yaw and pitch rateinformation is illustratively computed by differencing like sensorsacross the baseline(s). In embodiments that utilize three or more GNSSantenna/accelerometer triad units, yaw, pitch and roll information maybe directly observable from the differential accelerometer data acrossthe baseline(s). Illustratively, in such embodiments, at least three ofthe GNSS antenna/accelerometer triad units would be mounted in anorthogonal manner.

The accelerometer data is further integrated to obtain solutions betweenavailable GNSS solutions. These accelerometer based solutions are fedinto the Kalman filter to obtain navigation and location information.Further, the INS 305 may compute the a position, velocity and attitudenavigation of the rigid body from the specific forces acting on therigid body.

FIG. 4 is a flowchart detailing the steps of a procedure 400 forcomputing location information in accordance with an illustrativeembodiment of the present invention. The procedure begins in step 405where the system obtains GNSS location information. Illustratively, theGNSS information may be obtained by analyzing the appropriate GNSSsatellite signals received at antennas 110 A, B and processed by theGNSS subsystem 310. In accordance with alternative embodiments of thepresent invention, the GNSS subsystem 310 may include various features,such as, multipath detection, etc. that may be utilized to improve theGNSS location information.

Inertial motion unit information is then obtained in step 410. This maybe obtained by collecting accelerometer data from the accelerometertriads 115 A, B. The rotation rate is then obtained in step 415. Therotation rate may be obtained by analyzing the forces measured along therigid body from the two accelerometer triads 115A, B. Roll and pitchinformation may be computed directly from the accelerometer data. TheINS then removes the effects of gravity and other errors to obtain ameasurement of the acceleration and rotations acting on the rigid body.This rotational information may then be utilized for navigation/locationpurposes.

Illustratively, the mechanization process may be utilized to convert theraw accelerometer data into navigation information. This mechanizationprocess illustratively uses the conditions associated with the endingboundary of the previous measurement interval, and propagates theposition, velocity and attitude to the end of the current measurementinterval. Illustratively, is done using the delta velocities and deltaangles in the solution of the fundamental differential equations, as isknown to those skilled in the art and as are commonly illustrated bypublications involving INS/GNSS integration for geodetic applications:

$\frac{{dR}_{b}^{e}}{dt} = {R_{b}^{e}\left( {\Omega_{ei}^{b} + \Omega_{ib}^{b}} \right)}$And$\frac{d^{2}r^{e}}{{dt}^{2}} = {{R_{b}^{e}f^{b}} + {\mathcal{g}}^{e} - {2\Omega_{ie}^{e}\frac{{dr}^{e}}{dt}}}$The first differential equation maintains the attitude relationshipbetween the reference, or body, frame and the computational frame (ECEFin this case). The R_(b) ^(e) transformation matrix is maintained withthe following quaternion elements and is recomputed at the IMU samplingrate.

$R_{b}^{e} = {\begin{bmatrix}r_{11} & r_{12} & r_{13} \\r_{21} & r_{22} & r_{23} \\r_{31} & r_{32} & r_{33}\end{bmatrix} = {\quad\begin{bmatrix}{q_{1}^{2} - q_{2}^{2} - q_{3}^{2} + q_{4}^{2}} & {2\left( {{q_{1}q_{2}} - {q_{3}q_{4}}} \right)} & {2\left( {{q_{1}q_{3}} + {q_{2}q_{4}}} \right)} \\{2\left( {{q_{1}q_{2}} + {q_{3}q_{4}}} \right)} & {q_{2}^{2} - q_{1}^{2} - q_{3}^{2} + q_{4}^{2}} & {2\left( {{q_{2}q_{3}} + {q_{1}q_{4}}} \right)} \\{2\left( {{q_{1}q_{3}} + {q_{2}q_{4}}} \right)} & {2\left( {{q_{2}q_{3}} + {q_{1}q_{4}}} \right)} & {q_{3}^{2} - q_{1}^{2} - q_{2}^{2} + q_{4}^{2}}\end{bmatrix}}}$The second differential equation maintains the relative position andvelocity. The 2^(nd) order equation can be used to generate two firstorder equations by introducing velocity, v^(e).

$\frac{{dr}^{e}}{dt} = v^{e}$$\frac{{dv}^{e}}{dt} = {{R_{b}^{e}f^{b}} + {\mathcal{g}}^{e} - {2\Omega_{ie}^{e}\frac{{dr}^{e}}{dt}}}$In the equation for

$\frac{{dv}^{e}}{dt},$the effects of gravity and the Coriolis force may be removed from themeasured specific forces transformed to the computation (ECEF) frame bysubstitutingf ^(e) =R _(b) ^(e) f ^(b)The angular rates are derived from the basic rigid body kinematicequation using two points, P and Q as is described in Dynamics, Theoryand Applications, by Kane, T. R. and D. A. Levinson (1985).v ^(P) =v ^(Q) +ω×rThe angular acceleration, α, of the rigid body is determined by therelationship between the acceleration, a^(P), of P and the acceleration,a^(Q), of Q.a ^(P) =a ^(Q)+ω×(ω×r)+α×r

$v^{P} = {\frac{dp}{dt} = {{\frac{d}{dt}\left( {q + r} \right)} = {{\frac{dq}{dt} + \frac{dr}{dt}} = {v^{Q} + {\omega \times r}}}}}$$a^{P} = {\frac{{dv}^{P}}{dt} = {{\frac{{dv}^{Q}}{dt} + {\frac{d\;\omega}{dt} \times r} + {\omega \times \frac{dr}{dt}}} = {a^{Q} + {\alpha \times r} + {\omega \times \left( {\omega \times r} \right)}}}}$Where,

a^(X)—acceleration of point X, in the b-frame,

α—angular acceleration vector of body,

r—position vector of point P relative to Q, in the b-frame, and

ω—angular velocity of the body.

The location information is then output to a Kalman filter step 420.That is, GNSS information, the accelerometer information and thecomputed information from the accelerometer information (e.g., rotationrate, etc.) are fed into the Kalman filter. Lastly, the Kalman filterutilizes the various input information to generate location informationthat is an output for use by other components (not shown). The procedurethen loops back to step 405 for the next iteration.

It should be noted that while this invention has been described in termsof feeding the accelerometer and related information into a Kalmanfilter for processing, the principles of the present invention may beutilized in other environments. As such, the system described hereinshould be taken as exemplary only. It is expressly contemplated that theprinciples of the present invention may be utilized in systems withaccelerometer triads mounted to a rigid body but not integrated with aKalman filter, etc.

While the present invention has been described in terms of hardware, orof various components performing certain operations, it should be notedthat these various procedures may be implemented in hardware, software,firmware or a combination thereof. Therefore, be description of certainelements being performed in software, hardware, etc. should be taken asexemplary only. Further, as will be appreciated by those skilled in theart, variations for alternative embodiments of those described hereinmay be utilized without departing from the spirit and/or scope of thepresent invention.

What is claimed is:
 1. A system comprising: a first assembly comprisinga first global navigation satellite system (GNSS) antenna operativelyassociated with a first accelerometer set; a second assembly comprisinga second GNSS antenna operatively associated with a second accelerometerset; a rigid body that is operatively connected to the first assemblyand the second assembly, wherein the first and second assemblies areseparated by a predefined distance, whereby movement of the rigid bodycauses both the first and second assemblies to move; and a processorconfigured to computer a baseline vector between the first and secondassembly and further configured to compute rotational information usingdata from the first and second accelerometer sets.
 2. The system ofclaim 1 wherein the first accelerometer set comprises a trio oforthogonally oriented accelerometers.
 3. The system of claim 1 whereinthe second accelerometer set comprises a trio of orthogonally orientedaccelerometers.
 4. The system of claim 1 wherein the processor isfurther configured to compute the baseline vector using carrier phaseobservations.
 5. The system of claim 1 wherein the processor is furtherconfigured to compute roll, pitch and yaw information from data from thefirst and second accelerometer sets.
 6. The system of claim 1 whereinthe processor calculates the rotational information after elimination oferrors from the data from the first and second accelerometer sets. 7.The system of claim 1 further comprising: a third assembly comprising athird global navigation satellite system (GNSS) antenna operativelyassociated with a third accelerometer set; wherein the rigid body isoperatively connected to the thirds assembly, wherein the firstassembly, the second assembly and the third assembly are arranged in anorthogonal manner, whereby movement of the rigid body causes the first,second, and third assemblies to move; and wherein the processor isfurther configured to compute the rotational information using data fromthe first, second, and third accelerometer sets.
 8. A system comprising:a plurality of assemblies, each of the plurality of assembliescomprising a global navigation satellite system (GNSS) antennaoperatively associated with an accelerometer set; a rigid body that isoperatively connected to the plurality of assemblies, wherein each ofthe plurality of assemblies are separated are separated by a predefineddistance, whereby movement of the rigid body causes each of theplurality of assemblies to move; and a processor configured to computerbaseline vectors among the plurality of assemblies and furtherconfigured to compute rotational information using data from theplurality of accelerometer sets.